EATON TN03 520-1002 MTL Zirconia Oxygen Analysers Instructions
- June 12, 2024
- EATON
Table of Contents
CROUSE-HINDS SERIES
Technical note
MTL gas analysers & systems
TN03 520-1002 MTL Zirconia Oxygen Analysers
The Use of Zirconia Oxygen Analysers in Heat Treatment
The MTL Zirconia Oxygen analysers can be used to measure the properties of
a heat treatment atmosphere (carburising, annealing etc.) by measuring the
amount of oxygen in them.
In a carburising furnace, a hydrocarbon, typically natural gas or propane, is
“cracked” to provide the atmosphere. The “cracking” is really burning with too
little oxygen, so that not all the carbon and hydrogen in the fuel gas is used
up. The equations below illustrate this, using methane (natural gas) as fuel.
Stoichiometric combustion (stoichiometric means exactly the right amount of
one chemical to react with another) looks like this: CH4+ 2O2 = CO2+ 2H2O
But if you “crack” the fuel with too little oxygen, you get this: 4CH4+ 4O2=
2CO + 2CO2+ 2H2O + 6H2
However, this is only one possible reaction. Depending on temperature, you
could get: = CO + 3CO2+ H2O + 7H2
or even = 3CO + CO2+ 3H2O + 5H2
The other effect to be considered is dissociation, or the breakdown of a
molecule when heated. It is a reversible reaction and the equation must remain
numerically balanced at a particular temperature. Both carbon dioxide and
water will dissociate at the high temperature of our analyser, and the
resulting output from the cell would be due to the oxygen from this break-
down:
2H2O = 2H2+ O2. . . . . . (i)
2CO2= 2CO + O2. . . . . . .(ii)
Both water and carbon dioxide dissociate equally at one particular
temperature, 812°C. As the equations must remain numerically balanced at a
given temperature – if you increase the amount of (say) carbon monoxide in
equation (ii), some of the oxygen will be used up to convert it to carbon
dioxide. So the amount of oxygen present measures the ratio between carbon
dioxide and carbon monoxide, and between water and hydrogen, which are both
the same at 812°C. Oxygen is proportional to H2O/H2 and CO2/CO
The general formula for the cell output at 812°C is: O/P (mV) = 950 – 107.7
log
So a measure of oxygen made at 812°C will tell you the combined ratios of
oxides to fuels, directly. At any other temperature, you also need to know the
carbon/hydrogen ratio of fuel.
Referring back at the three equations for excess methane and oxygen. If you
count up the molecules of oxide gases and divide by the molecules of fuel
gases, you will find that the ratio is 1:2 in all cases. So it does not matter
just how the methane is cracked – with a particular amount of oxygen – we will
always finish up with the same ratio of oxides to fuels.
The significance of this ratio is that it determines how much carburising
potential a gas has. But the water and hydrogen play their parts too, because
too much water will provide some oxygen that will combine with the carbon
monoxide to form carbon dioxide, and it is the carbon monoxide that does the
carburising.
So now our single measurement can replace three “traditional” measurements;
those of carbon monoxide, carbon dioxide and dewpoint. Our single oxides-to-
fuels measurement tells the user all that is required – but the user may feel
that it cannot be interpreted in the accustomed fashion.
So for practical purposes, we can ignore the hydrogen/ water break-down, and
concentrate on the carbon monoxide/ carbon dioxide. The graph shows cell
output against carbon monoxide/carbon dioxide ratio. This is plotted at 634°C
and 812°C – being the two principal temperatures that our analysers are
operated at – although for metallurgical processes 812°C is the more usual. We
find however that more often than not users “calibrate” our analyser output
against what they regard as ‘good quality’ products, and no longer rely on the
traditional interpretation of carbon monoxide or carbon dioxide. All the user
has to remember is that the higher the analyser output, the more carbon
monoxide and hydrogen is present, and the lower the output, the more carbon
dioxide and water.
MTL Zirconia analysers can be scaled to read out in kilocalories (oxygen
potential) or oxide-to-fuel ratio. There is also an empirical approach; obtain
the readout in millivolts, and establish upper and lower readings by reference
to product quality; the furnace operator has then only to keep the analyser
reading between those limits. For automatic operation, we can supply
adjustable limit switches to do the same thing.
Note: The MTL Z1110 analyser is used in this type of application.
OTHER TECHNICAL NOTES
| TN01 | Oxygen Sensors – Theory and Application |
|---|---|
| TN02 | “Using MTL Zirconia Oxygen Analysers to Measure the Dewpoint of |
Furnace Atmospheres”
The given data is only intended as a product description and should not be regarded as a legal warranty of properties or guarantee. In the interest of further technical developments, we reserve the right to make design changes.
Eaton Electric Limited,
Great Marlings, Butterfield, Luton
Beds, LU2 8DL, UK.
Tel: + 44 (0)1582 435600
Fax: + 44 (0)1582 422283
www.mtl-inst.com
E-mail:mtlgas@eaton.com
EUROPE (EMEA):
+44 (0)1582 723633
mtlenquiry@eaton.com
THE AMERICAS:
+1 800 835 7075
mtl-us-info@eaton.com
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+65 6 645 9888
sales.mtlsing@eaton.com
© 2016 Eaton
All Rights Reserved
Publication No. TN03 520-1002 Rev 3 191016
October 2016
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EATON TN03 520-1002 MTL Zirconia Oxygen
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